Electrophotographic copysheet



United States Patent 01 Rice 3,331,596 ELECTRDPHOTOGRAPHIC COPYSHEET Larry J. Bresina, St. Paul, Minn Einar I). Horne, Hud- This invention relates to improved electrophotograp hic copysheet constructions, particularly to constructi ns which have been developed electrolytically to provide color images thereon. In one aspect this invention relates to an electrophotogra-phic sheet construction having a full color print thereon and being stabilized to prevent color shift of the dyestuffs. In still another aspect, this invention relates to a stabilized color electrophotographic print having a protective coating.

Recent developments in electrophot-ography, both in electrostatic and in electrolytic imaging processes, ha e made possible the preparation of full color reproductions from light images. In the field of electrolytic electrophotogra-phy successive deposits of subtractive primary colors may be applied to the surface of an electrolytically develop'able copysheet, such as the copysheet f US. 3,010,884, to provide the desired full color print. Suitable color developers include basic nitrogen containing organic compounds which are soluble in an aqueous acidic solution and which are in soluble in water at a pH above 7, as is described in US. Ser. No. 23,017, now US.

3,172,827, and water soluble oni um cations which decompose electrolytically to form water insoluble com pounds, as is disclosed in US. Ser. No. 22,931, now US. 3,172,826. Many of the preferred developers are colored dyestuffs which are photoreducible in the a sence of oxygen. Attempts to provide a transparent protective or decorative coating over the final full color print have fre quently resulted in a shift of color value upon extended exposure of the print to ordinary light, resulting in print instability. Some of the most desirable dyestufis are unstable when protective films or coatings over the colored print serve to isolate the dyestuffs from oxygen in the atmosphere.

It is therefore an object of this invention to provide a means for initially stabilizing a freshly prepared color image on an electrophotographic print.

Another object of this invention is to provide a stabilized color electrophotographic print which contains a transparent protective layer thereon.

A further object of this invention is to provide a transparent film which can be applied to a col-or electrop-hotographic print and which tends to stabilize photoreducible dyestuffs against color shifting.

Various other objects and advantages will become apparent from the following disclosure.

It has been now found that a substantially air impermeable, light transparent layer can be placed over the image containing surface of an electrophotographic copysheet, the image containing a colored dyestuff capable of photoreduction in the presence of zinc oxide and in the absence of oxygen, provided the interfacial areas between the transparent layer and the copysheet surface contain a colorless peroxide oxidizing agent, preferably an organic or inorganic hydroperoxide. Freshly prepared colored prints protected in this manner are more resistant to a change in color value during the initial period of exposure to ordinary light, when such color shifting is most noticeable, and are further protected by the superimposed layer.

Patented May 7, 1-968 The transparent layer can be prepared from any of the substantially air impermeable materials, including both natural and synthetic polymers, organic as well as inorganic. Thin transparent layers of zinc sulfide, silica and titania can be provided, e.g. see US. 2,886,434. Glass and any of the substantially air impermeable plastics, such as polyethylene terephthalate, high density polyethylene, polyvinylidene fluoride, chlorotrifiuoroethylene vinylidene fluoride copolymer, polycarbonate, polyvinylidene chloride, polyurethane, polyvinyl alcohol, polyamides, etc. which form transparent coatings may be used. Although the preferred coating materials are substantially impermeable to air, the more permeable materials can suitably be employed at the higher coating thicknesses or in conjunction with other materials that reduce airpermeability. Preferably the oxygen transmission rate of the coating layer should be below about 200, most preferably below about 100, cc./ 100 in. /24 hr./atm./mil. The coating may be prepared on the image containing electrophotographic sheet by knife or extrusion coating, by simply painting the sheet surface or by vapor deposition. In one preferred method a preformed film or sheet of the coating material is laminated or adhered, e.g. with a pressure sensitive or heat activated adhesive, to the surface of the electrophotographic print. The oxidizing agent may be applied by coating the image-containing surface directly with the oxidizing agentlbefore superimposing the transparent protective coating thereon or by incorporating the oxidizing agent into the adhesive used to laminate the protective coating to the print surface. When a preformed protective film is used which can be bonded onto the print surface without the use of an adhesive, the oxidizing agent can optionally be carried on that film surface which is brought into contact with the image-containing surface of the print.

Any colorless oxidizing agent which is normally non gaseous and which provides an oxidizing environment can be employed, although the inorganic peroxides, e.g. hydrogen peroxide, and organic peroxides, especially hydroperoxides such as 2,S-dimethylhexane-Z,S-dihydroperoxide, are preferred.

Only electrophotogranhic copysheets prepared by electrostatic or electrolytic techniques and having colored images which contain a dyestulf capable of photoreduction in the presence of zinc oxide and in the absence of oxygen are contemplated within the scope of this invention. Many of the amino dyestuffs described in US. Ser. No. 23,017 are of this type. Such dyestuffs can readily be identified by the following simple test procedure.

A small amount of the dry powdered dyestulf is mixed With about an equal volume of zinc oxide powder. Sufficient ethanol is added to the mixture to produce a thick slurry. Several drops of triethylamine are added to the slurry, and the slurry is then spread onto the surface of a glass plate. A glass plate is placed over the slurry coated surface, and the edges of the resulting sandwich are taped to prevent air from entering the slurry coating. The slurry coating is exposed through one of the glass plates for one hour with a 275 Watt sunlamp placed at a distance of one foot from the plate surface. If a visible difference in color is observed at any time during the exposure period which, on re-exposure to air, is converted back to the original color, the dyestuff is photoreducible as defined earlier.

The following examples will serve to illustrate the invention without limiting the scope thereof.

A 0.00075 inch thick polyethylene terephthalate film was coated with the following heat scalable adhesive (30 weight percent solids in acetone).

2774) 9.9 Citric acid 0.1 Peroxide compound (equivalents) 0.08 10 The dry coating weight was about 2 grams per square foot. The adhesive side of this sheet construction was placed into contact with the image containing surface of an electrolytically developed photoconductive zinc oxide copysheet of the type described in US. 3,010,884. The 15 image deposits contained a yellow photoreducible dyestuff of the formula The composite was then passed through heated squeeze rolls to bond the two sheets and provide a transparent protective layer over the imaged copysheet. The peroxides in Table I were incorporated into the above formulation. Each laminated sample was irradiated with sunlamps at an intensity level of 700 foot-candles and was rated after one hour exposure according to its ability to prevent a colorshift of the dye.

TABLE I Half life Peroxide Rating (hrs. at 130 C (1) None (control Very poor, turned blown. (2) Tertiary butyl hydroperoxide. Very good 520 (3) Mixture of methyl ethyl d0 1. 2

ketone peroxides and hydroperoxides. (4) Cumene hydroperoxide "do 113 (5) 2,5-dimethylhexyl-2,5- Good 67 dihydroperoxide. (6) p-Methane hydroperoxide .110 12. 5 (7) Tertiary butyl perbenzoate Fair 0.55 (8) Dicurnyl peroxide oticeable reduction 1. 84

of colorshilt. (9) Ditertiary butyl peroxide do 6. 4 (l0) 2,5-dimethyl-2,5-di (tertiary d0 8.2

butyl peroxy) hexyner3.

From the results in Table I it can be seen that the hydroperoxides are more elfective than the peroxides. Hazardous oxidizing agents, i.e. those with a minimum half life below about 0.4, are preferably not used.

To illustrate the use of an inorganic oxidizing agent the following test was performed. The image-containing surface of an electrolytically developed copysheet similar to that earlier described was thoroughly wetted with a 6' weight percent solution of hydrogen peroxide in water. A transparent laminating film of the type described above, except for the omission of peroxide in the adhesive formulation, was laminated to the wetted surface with the same squeeze roll technique. After a two hour exposure to sunlamps the yellow image on the control has turned brown whereas the sample containing the hydrogen peroxide had virtually no color change.

Other embodiments and modifications will become apparent to those skilled in the art without departing from the spirit or scope of this invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrophotographic copysheet having on one surface thereof an imagewise distribution of a colored dyestuft capable of photoreduction with a color change in the presence of zinc oxide and in the absence of oxygen, and a substantially air impermeable, light transparent coating thereon, the interfacial areas between said transparent coating and said copysheet surface containing a colorless peroxide oxidizing agent.

2. The electrophotographic copysheet of claim 1 wherein said colorless peroxide is a hydroperoxide.

3. The electrophotographic copysheet of claim 1 wherein said colorless peroxide is hydrogen peroxide.

4. The electrophotographic copysheet of claim 1 where in said colorless peroxide is tertiary butyl hydroperoxide.

5. The electrophotographic copysheet of claim 1 wherein said colorless peroxide is cumene hydroperoxide.

6. The electrophotographic copysheet of claim 1 wherein said colorless peroxide is 2,5'dimethylhexy1-2,S-dihydroperoxide. 1

7. The electrophotographic copysheet of claim 1 wherein said colorless peroxide is p-menthane hydroperoxide.

8. The electrophotographic copysheet of claim 1 wherein the interfacial areas between said transparent coating and said copysheet surface also contain a colorless adhesive.

References Cited UNITED STATES PATENTS 2,661,289 12/1953 Mayo et al 961 X 3,083,117 3/1963 Schmiedel et a1. l17-17.5 3,291,606 12/1966 'Jeilreys 951 X NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner. 

1. AN ELECTROPHOTOGRAPHIC COPYSHEET HAVING ON ONE SURFACE THEREOF AN IMAGEWISE DISTRIBUTION OF A COLORED DYESTUFF CAPABLE OF PHOTOREDUCTION WITH A COLOR CHANGE IN THE PRESENCE OF ZINC OXIDE AND IN THE ABSENCE OF OXYGEN, AND A SUBSTANTIALLY AIR IMPERMEABLE, LIGHT TRANSPARENT COATING THEREON, THE INTERFACIAL AREAS BETWEEN SAID TRANSPARENT COATING AND SAID COPYSHEET SURFACE CONTAINING A COLORLESS PEROXIDE OXIDIZING AGENT. 